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We build multi million dollar homes in Phoenix AZ. This is a deseret community and we build slab on grade. 90% of our competitors use underground air returns with rigid duct supply in the attic. We insulate just below the roof, not at the ceiling, so our attic space is a bit more tolerable. We have chosen to install our a/r and supply ducting, flex, in the attic. We would like to understand why our competition uses underground a/r. We have not been able to find anyone in the trade that can show us definitively that one system is better than the other. We strive in everything we do to provide the best products and building techniques for our customers. We are proud of our homes. We do not want to install underground a/r just because most of our competition is doing so. Is there any hard data available showing the pros and cons of each, especially regards the Phoenix AZ area. The idea of returning the air in the cool underground is appealing, but that is not hard data.

advantage: pipe should be cooler; maybe shorter= excellent! the laws of physics are not location specific -- they work wherever.

having cooler pipes will probably not make much difference, but will not be a negative factor either. putting the pipe underground usually means that the joints will be better sealed.

what piping material is being used?

why do you not install supply ducts within the conditioned space? doing that would be one item to keep you on the leading edge. or, are the attics sealed? if so, the supplies are within conditioned space, the attic just adds to the conditioned volume. are you insulating the supply pipes? to what R value?

my Dad had "perimeter" FA heating installed in 1947 in LaPorte IN = snow country -- then a/c added in late 1960s -- Transite supply ducts -- still working when house sold ~1997. nice having warm floors, mostly tile on the slab.